1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for controlling a power amplifier.
2. Description of the Related Art
In a communication system such as a bandwidth-efficient digital system using Quadrature Amplitude Modulation (QAM) or a Frequency Demodulation Multiplexing (FDM) communication system using multi-carrier or single sideband (SSB) signals, a signal is subject to modulation and multiplexing and thus the time-varying envelope of its Peak to Average power Ratio (PAR) varies greatly. A base station (BS) uses a power amplifier (PA) with good linearity to amplify a Radio Frequency (RF) signal prior to transmission in the communication system.
A cellular system such as Code Division Multiple Access CDMA or Orthogonal Frequency Division Multiplexing (OFDM) transmits a modulated multiplexed signal having a high PAR to multiple users sharing the same frequency band. Radio Frequency Power Amplifiers (RFPA) used in conventional communication systems use power inefficiently because they consume a large amount of Direct Current (DC) to amplify the RF signal having a high PAR and are expensive to manufacture.
In order to increase the efficiency of a PA, the power drawn from a power supply for the PA is adjusted according to the size of a signal envelope. This principle is called bias adaptation. This bias control scheme controls DC bias according to the envelope of an input signal into a transistor in order to reduce power consumption in the transistor.
FIG. 1 is a block diagram illustrating an example of a conventional power amplification control apparatus using a general bias control scheme.
Referring to FIG. 1, the power amplification controlling apparatus comprises an envelope detecting circuit (EDC) 101 for detecting the envelope of an input signal and a voltage upconverter (VUC) 102 for upconverting a DC voltage Vc received from a DC supply 103 which is a system voltage supply based on the signal envelope.
When the voltage of the input signal envelope exceeds a predetermined threshold, the VUC 102 increases the power supply voltage. Therefore, the change of mean input power varies the characteristics of an RFPA 104. Also, the characteristics of the RFPA 104 and the EDC 101 vary with temperature changes. As a result, the RFPA 104 cannot perform optimum amplification.
Unlike a PA in a terminal, a PA in a BS consumes hundreds of watts and thus requires a VUC of hundreds of watts. However, the manufacture of such a VUC is not viable because of manufacturing costs and technological constraints. As a solution to this problem, the power amplification control apparatus illustrated in FIG. 2 was proposed.
FIG. 2 is a block diagram illustrating an example of a conventional power amplification controlling apparatus using an improved bias control scheme. The improved bias control scheme uses two DC voltages instead of one DC voltage.
Referring to FIG. 2, the power amplification controlling apparatus comprises a first DC supply 203 for supplying a power supply voltage Vc, a voltage combiner (VC) 205 for combining Vc with a voltage Vv received from a VUC 202, and a second DC supply 206 for supplying a voltage Max Vv to the VUC 202. The VUC 202 and the VC 205 collectively form a voltage enhancement circuit (VEC) 207.
The first DC supply 203 supplies a predetermined constant DC voltage Vc to an RFPA 204 and the VUC 202 changes the voltage max Vv to the voltage Vv between 0 and Max Vv according to the envelope of a signal input to the RFPA 204. The second DC supply 206 outputs Max Vv which is a maximum value of the output voltage Vv of the VUC 202.
In operation, if the voltage of the input signal envelope is equal to or less than a predetermined threshold, Vc is supplied to the RFPA 204. If the signal envelope voltage exceeds the threshold, the VC 202 adds Vc to Vv and supplies the sum as a bias voltage Vp (Vp=Vc+Vv) to the RFPA 204. For supplying Vc, an existing power supply of hundreds of watts is used. However, the VUC 202 for supplying the voltage Vv varying with the input signal envelope can be implemented at tens of watts.
The relationship among the power supply voltage Vc, the break-down voltage Vb of a transistor, the VUC output voltage Vv, and the bias voltage Vp over the signal envelope is illustrated in FIG. 3.
In the power amplification controlling apparatus illustrated in FIG. 2, Max Vv must be (Vb−Vc) to protect the transistor. In other words, Max Vv must be determined according to Vb. However, Vc varies due to the use of an auxiliary power supply in the case of an electrical failure in a BS, or due to power discharge in a terminal. Therefore, with Max Vv fixed to (Vb−Vc), if Vc rises by ΔVc, Vp=Vc+ΔVc+Vv>Vb, it leads to deterioration of the transistor characteristics or the destruction of the transistor. If Vc drops by ΔVc, Vp=Vc−ΔVc+Vv<Vb, the resulting distortion of the input signal to the RFPA 204 leads to deterioration of the spurious characteristics of the input signal.